Algicidal effects of Alternanthera philoxeroides mediated by the nutrient environment in a shallow, eutrophic lake (Chaohu Lake, China)

We investigated the nitrogen and phosphate nutrient status in the habitats of Alternanthera philoxeroides and its co-occurring algal growth in a field survey. We also determined the algicidal activity of A. philoxeroides extracts from a diversity of habitats in Chaohu Lake, China, in the laboratory. We then tested the correlation between the nutrient status and the algicidal activity of A. philoxeroides. The aim was to elucidate whether environment induced algal inhibition by aquatic A. philoxeroides. The results showed that the cell numbers of Chlorella pyrenoidosa and Microcystis aeruginosa were significantly suppressed by A. philoxeroides. The overall nutrient range where allelopathic inhibition of algae occurred with A. philoxeroides was very broad, suggesting that the capacity of A. philoxeroides for the allelopathic inhibition of algae was not dependent on the nutrient status of water bodies. Furthermore, A. philoxeroides had different allelopathic inhibitory effects, which in order of descending rank were: C. pyrenoidosa > M. aeruginosa > natural phytoplankton assemblage. Our experiments suggest that nitrogen and phosphate induced the algal growth inhibitory effects of A. philoxeroides.   Key words: Alternanthera philoxeroides, aquatic ecosystem, algicidal effects, environmental induction, butterfly curve.